That is a good question, Herb. The physical evidence seems to show they were first used in late 1943 or early 1944 as some of the components on my generator assembly are dated in October 1943. I have photos and a Willys Overland engineering drawing of a later data plate specifically for the 24v generator as "a unit of Model MZ series radio equipment", dated 12-27-44.
I do have a 12/24v NOS power supply with the inspection tag still on it dated 3-20-1945.
Sorry to say I don't have a BOM on this item yet so I don’t really know if any jeeps came out of the factory with this option.